NEW DELHI: Veteran submariners are suggesting that the Navy should not rule out the possibility of sabotage in the sinking of INS Sindhurakshak, even as indications are emerging that the vessel was preparing for "a war patrol" with armed torpedoes and cruise missiles close to Pakistani shores.

Their suggestion that the just-ordered naval board of inquiry (BoI) — chaired by a senior officer from the submarine wing — should thoroughly probe the sabotage angle stems from the fact that warheads of the torpedoes and missiles have several layers of security features built in to avoid inadvertent blasts aboard the vessel.

"The warheads, whether heavy torpedo warheads or relatively smaller missile warheads, are designed with highest levels of inbuilt safety. These multi-layered measures added together ensure almost foolproof levels of safety," said Dean Mathew, a former Navy Commander and research fellow at Institute for Defence Studies and Analyses (IDSA), who was a guided weapons specialist.

Mathew and several other submarine veterans said the possibility of a "cook off", where warheads start exploding under high temperature, too, is almost impossible. Submarines like INS Sindhurakshak have mechanisms such as sprinklers that get activated in case temperature in the torpedo bay shoots up.

"So this should have worked if there was a fire in the battery compartment beneath and the temperature was shooting up in the torpedo bay above," a veteran said. For TNT, which is the main explosive filling in a torpedo, just high temperature is not enough to cause an explosion under normal circumstances. The exception could be if TNT is expired, and its chemical structure has become unstable.

Based on their own personal experiences, these veterans are arguing that explanations for the tragedy have to be more than simple answers of material failure or standard operating procedures (SOPs) not being followed.

Given the history of INS Sindhurakshak, the first explosion could be fuelled by hydrogen gas fuelled from the battery compartment. However, they suspect that the subsequent explosions — given the enormity and scale — are almost certainly some warheads — either of the combat torpedoes or missiles stored onboard. "The fact that these explosions ruptured the hulls of the submarine and sank it strongly points to the warheads exploding as they are meant to do exactly the same when used against an enemy submarine or ship," one former officer said.

Even when a submarine is out on a war patrol, the exploder unit, which triggers the warhead explosion in a torpedo, is stored separately from the warhead and assembled into it only on explicit orders from the commanding officer to "arm" the torpedo in preparation for an imminent war situation. The exploder unit is never mated with the torpedo in the harbour, while it is preparing for patrol.

The next level of safeties are that even with the exploder unit assembled, the unit gets active only once the torpedo is launched from the submarine and travelled out a safe distance away from the submarine. There are many more levels of inbuilt safety nets that make sure that the warhead doesn't go off just like that, veterans argue.

In case of missiles on board, their safety and arming unit (SAU) ensures that the warhead is not ready for explosion unless the missile is powered, launched and travelled a safe distance away. These information are fed to the missile by the submarine's combat information console, it can even be mimicked to make the missile feel that it is ready. If a missile has exploded, the question is, "did anyone get any of the missiles onboard 'ready' inadvertently or otherwise?" asks a submariner.

Another officer asked why was that the first explosion and fire couldn't be brought under control? "Clearly there is a human element that was involved directly or indirectly," he said. Another former submariner pointed out that the recent explosions onboard Indian submarines — on the same submarine and on INS Sindhuvijay — were both limited. On Sindhurakshak one sailor had lost his life in 2010 and in the case of Sindhuvijay an officer was injured. "The warheads doesn't go off like that," he argued.